The present invention relates to a method for separating isotopes of an element where the isotopes are present in the form of molecules and where the isotopes take part in the vibration of the molecules and can be selectively excited by means of radiation, and then separated.
Such a process is known from German Offenlegungsschrift No. 1,959,767, hereby incorporated by reference, but this process is limited to the separation of the U.sup.235 and U.sup.238 isotopes where the respective type of atom is used to form molecules which have an absorption characteristic (vibration-rotation line) corresponding to the emission line of a laser, preferably a CO.sub.2 laser. In this process, the uranium atom participates in the vibrations of the molecule (e.g., UF.sub.6 molecules containing U.sup.235 F.sub.6 molecules and U.sup.238 F.sub.6 molecules), a reaction mixture is produced by the addition to the isotopic molecules of one or a plurality of reaction partners to form a reaction mixture and this mixture is irradiated by the laser to excite the molecules isotope--specifically in their vibration-rotation spectra. A chemical reaction takes place and, upon completion of the chemical transformation of the irradiated portions of the reaction mixture, the various molecules are separated in a known manner, for example, by fractional condensation. Consequently, monochromatic light must be employed, so that only molecules containing the type of isotope in question are excited and no other molecules, i.e., the CO.sub.2 laser is selectively tuned to one line. However, since molecular spectra have a plurality of absorption lines, the irradiation of only one certain resonance line is able to selectively excite at most a very small fraction of all isotopes. It is therefore impossible to economically produce significant quantities of gas with selectively excited molecules. Moreover, at room temperature, the average line density of UF.sub.6, inter alia, is so great that, as a result of the natural width of the lines, spread by Doppler and pressure effects, the lines will overlap. The know spectra, for example, of UF.sub.6 indicate that the vibration-rotation bands of the UF.sub.6 molecules constitute a quasi continuum. If, therefore, a laser line is irradiated which coincides with an absorption line of the molecule to be selectively excited, the excitation will be selective only if in the quasi continuum of the other isotope there happens to be a gap, i.e., no absorption line. Such gaps have not as yet been found.
These drawbacks can be reduced to a limited degree by substantially lowering the temperature of the gas which decreases the number of the various molecular states, the line density, and the Doppler and pressure spreading. However, a lowering of the temperature has the simultaneous result that the vapor pressure, i.e., the molecular density of UF.sub.6, is greatly reduced. The chemical reactions to take place upon completion of the selective excitation can thus no longer be effected with the large quantities of gas required for economical operation of the process.